1. Field of the Invention
The present invention relates to a catalytic trap for treating exhaust gas streams, especially those emanating from lean-burn engines, and to methods of making and using the same. More specifically, the present invention provides a catalytic trap which abates NOx in the exhaust streams being treated and exhibits enhanced durability after aging at high temperature and lean operation conditions.
2. Related Art
Emission of nitrogen oxides (xe2x80x9cNOxxe2x80x9d) from lean-burn engines (described below) must be reduced in order to meet emission regulation standards. Conventional three-way conversion (xe2x80x9cTWCxe2x80x9d) automotive catalysts are suitable for abating NOR, carbon monoxide (xe2x80x9cCOxe2x80x9d) and hydrocarbon (xe2x80x9cHCxe2x80x9d) pollutants in the exhaust of engines operated at or near stoichiometric air/fuel conditions. The precise proportion of air to fuel that results in stoichiometric conditions varies with the relative proportions of carbon and hydrogen in the fuel. An air-to-fuel (xe2x80x9cA/Fxe2x80x9d) ratio of 14.65:1 (weight of air to weight of fuel) is the stoichiometric ratio corresponding to the combustion of a hydrocarbon fuel, such as gasoline, with an average formula CH1.88. The symbol xcex is thus used to represent the result of dividing a particular A/F ratio by the stoichiometric A/F ratio for a given fuel, so that xcex=1 is a stoichiometric mixture, xcex greater than 1 is a fuel-lean mixture and xcex less than 1 is a fuel-rich mixture.
Engines, especially gasoline-fueled engines to be used for passenger automobiles and the like, are being designed to operate under lean conditions as a fuel economy measure. Such future engines are referred to as xe2x80x9clean-burn enginesxe2x80x9d. That is, the ratio of air to fuel in the combustion mixtures supplied to such engines is maintained considerably above the stoichiometric ratio (e.g., at an air-to-fuel weight ratio of 18:1) so that the resulting exhaust gases are xe2x80x9cleanxe2x80x9d, i.e., the exhaust gases are relatively high in oxygen content. Although lean-burn engines provide enhanced fuel economy, they have the disadvantage that conventional TWC catalysts are not effective for reducing NOx emissions from such engines because of excessive oxygen in the exhaust. The prior art discloses attempts to overcome this problem by operating lean-burn engines with brief periods of fuel-rich operation. (Engines which operate in this fashion are sometimes referred to as xe2x80x9cpartial lean-bum enginesxe2x80x9d.) It is known to treat the exhaust of such engines with a catalyst/NOx sorbent which stores NOx during periods of lean (oxygen-rich) operation, and releases the stored NOx during the rich (fuel-rich) periods of operation. During periods of rich (or stoichiometric) operation, the catalyst component of the catalyst/NOx sorbent promotes the reduction of NOx to nitrogen by reaction of NOx (including NOx released from the NOx sorbent) with HC, CO and/or hydrogen present in the exhaust.
The use of NOx storage (sorbent) components including alkaline earth metal oxides, such as oxides of Ca, Sr and Ba, alkali metal oxides such as oxides of K, Na, Li and Cs, and rare earth metal oxides such as oxides of Ce, La, Pr and Nd in combination with precious metal catalysts such as platinum dispersed on an alumina support, is known, as shown for example, at column 4, lines 19-25, of U.S. Pat. No. 5,473,887 of S. Takeshima et al, issued on Dec. 12, 1995. At column 4, lines 53-57, an exemplary composition is described as containing barium (an alkaline earth metal) and a platinum catalyst. The publication Environmental Catalysts For A Better World And Life, Proceedings of the 1st World Congress at Pisa, Italy, May 1-5, 1995, published by the Societa Chimica Italiana of Rome, Italy has, at pages 45-48 of the publication, an article entitled xe2x80x9cThe New Concept 3-Way Catalyst For Automotive Lean-Bum Engine Storage and Reduction Catalystxe2x80x9d, by Takahashi et al (below referred to as xe2x80x9cthe Takahashi et al paperxe2x80x9d). This article discloses the preparation of catalysts of the type described in the aforementioned Takeshima et al U.S. Pat. No. 5,473,887 and using these catalysts for NOx purification of actual and simulated exhaust gases alternately under oxidizing (lean) and reducing (rich or stoichiometric) conditions. The conclusion is drawn in the last sentence on page 46, that NOx was stored in the catalyst under oxidizing conditions and that the stored NOx was then reduced to nitrogen under stoichiometric and reducing conditions. A similar but more detailed discussion is contained in SAE Paper 950809 published by the Society of Automotive Engineers, Inc., Warrendale, Pa., and entitled Development of New Concept Three-Way Catalyst for Automotive Lean-Burn Engines, by Naoto Miyoshi et al, was delivered at the International Congress and Exposition, Detroit, Mich., Feb. 27-Mar. 2, 1995.
U.S. Pat. No. 4,742,038, xe2x80x9cMonolithic Catalyst Support and Catalyst Deposited on the Supportxe2x80x9d, issued May 3, 1988 to S. Matsumoto, discloses a metal substrate for carrying a catalytic material useful for the treatment of exhaust gases from internal combustion engines.
U.S. Pat. No. 5,874,057, xe2x80x9cLean NOx Catalyst/Trap Methodxe2x80x9d, issued on Feb. 23, 1999 to M. Deeba et al, discloses a method of NOx abatement utilizing a composition comprising a NOx abatement catalyst comprising platinum and, optionally, at least one other platinum group metal catalyst which is kept segregated from a NOx sorbent material. The NOx sorbent material may be one or more of oxides, carbonates, hydroxides and mixed oxides of one or more of lithium, sodium, potassium, rubidium, osmium, magnesium, calcium, strontium and barium.
Prior art catalysts as described above have a problem in practical application, particularly when the catalysts are aged by exposure to high temperatures and lean operating conditions, because after such exposure, such catalysts show a marked decrease in catalytic activity for NOx reduction, particularly at low temperature (250 to 350xc2x0 C.) and high temperature (450 to 600xc2x0 C.) operating conditions.
U.S. Pat. No. 5,451,558, xe2x80x9cProcess For the Reaction and Absorption of Gaseous Air Pollutants, Apparatus Therefor and Method of Making the Samexe2x80x9d, issued on September 19, 1995 to L. Campbell et al, (xe2x80x9cthe Campbell et al Patentxe2x80x9d) discloses a catalytic material for the reduction of NOx from a turbine in a power generating stack, although the patent also refers at column 1, lines 13-14, generally to a process and apparatus for reducing pollutants xe2x80x9cwhich are produced by combustion of hydrocarbons or hydrogen in an engine or boiler, and primarily in a gas turbine.xe2x80x9d As disclosed at column 2, lines 23-37, the turbine exhaust gases are cooled to the range of 250 to 500xc2x0 F (about 121 to 260xc2x0 C.) before contacting the catalytic/adsorbent material (column 2, lines 23-37) and the oxidation is stated (column 2, lines 45-48) to occur at temperatures in the range of 150 to about 425xc2x0 F. (66 to 218xc2x0 C.), most preferably in the range of 175 to 400xc2x0 F. (about 79 to 204xc2x0 C.). The catalytic species comprises an oxidation catalyst species which may comprise various metals including platinum group metals (see column 3, line 67 through column 4, line 3) deposited on a high surface area support which may be xe2x80x9cmade of alumina, zirconia, titania, silica or a combination of two or more of these oxides.xe2x80x9d The catalyst-containing high surface area support is coated with an adsorbent species which may comprise xe2x80x9cat least one alkali or alkaline earth compound, which can be a hydroxide compound, bicarbonate compound, or carbonate compound, or mixturesxe2x80x9d thereof. At column 3, lines 16-22, the xe2x80x9ccarbonate coatingxe2x80x9d is said to be a xe2x80x9clithium, sodium, potassium or calcium carbonate, and presently the preferred coating is a potassium carbonate.xe2x80x9d At column 4, lines 28-31, however, it is stated that the absorber comprises xe2x80x9cmost preferably sodium carbonate, potassium carbonate or calcium carbonate.xe2x80x9d The high surface area support containing the oxidation species and adsorbent may be coated onto xe2x80x9ca ceramic or metal matrix structurexe2x80x9d as a carrier. See column 4, lines 12-20. The catalytic material is applied to the carrier by coating the carrier with, e.g., platinum-impregnated alumina, and then wetting the alumina with an alkali or alkaline earth carbonate solution, and then drying the wetted alumina (see column 5, line 9 through column 6, line 12). The carrier may be alumina beads as illustrated in FIG. 1A, or a monolithic ceramic or stainless steel support as illustrated in FIG. 1C, both Figures being described at column 4, line 67, to column 5, line 8. The use of a metal monolith support for the catalytic/adsorbent material is suggested at column 5, lines 48-58. There is no suggestion in the Campbell et al Patent of criticality of, nor is any importance assigned to, the type of substrate or high surface area support to be used with a particular adsorbent species. In fact, as noted above, silica is one of four high surface area supports taught for use with compositions preferably including a potassium carbonate adsorbent.
Generally, the present invention relates to a catalytic trap member having coated thereon a catalytic trap material comprising a catalytic component effective for the reduction of NOx and a NOx sorbent essentially comprising a basic oxygenated compound of an alkali metal selected from the group consisting of lithium, sodium and potassium, the catalytic material being coated onto a carrier member which is inert to the basic oxygenated compound.
Specifically, in accordance with the present invention there is provided a catalytic trap for conversion of NOx in an exhaust gas stream which is periodically alternated between (1) lean and (2) stoichiometric or rich conditions, the catalytic trap comprising the following components: (a) a catalytic trap material which comprises (i) a refractory metal oxide support having dispersed thereon a catalytic component effective for promoting the reduction of NOx under stoichiometric or rich conditions of the exhaust stream, and (ii) a NOx sorbent effective for adsorbing NOx under lean conditions of the exhaust gas stream and desorbing NOx under stoichiometric or rich conditions of the exhaust gas stream and comprising one or more basic oxygenated compounds of an alkali metal selected from the group consisting of lithium, sodium and potassium, the catalytic trap material being inert to the aforesaid basic oxygenated compounds, and (b) a refractory carrier member which is inert to the aforesaid basic oxygenated compounds and on which the catalytic trap material is coated.
In one aspect of the present invention the refractory carrier member may be selected from the group consisting of (a) refractory metal, e.g., stainless steel, Fecralloy or titanium, or (b) refractory oxides such as alumina, titania, zirconia, zirconia-alumina, zirconia-titania, titania-alumina, lanthana-alumina, baria-zirconia-alumina, niobia-alumina, and silica-leached cordierite.
In accordance with another aspect of the present invention, the catalytic trap for the above-described purpose comprises the following components: (a) a catalytic trap material which is substantially free of silica components and comprises (i) a refractory metal oxide support having dispersed thereon a catalytic component effective for promoting the reduction of NOx under stoichiometric or rich conditions of the exhaust stream, and (ii) a NOx sorbent effective for adsorbing NOx under lean conditions of the exhaust gas stream and desorbing NOx under stoichiometric or rich conditions of the exhaust gas stream, the NOx sorbent comprising one or more basic oxygenated compounds of an alkali metal selected from the group consisting of lithium, sodium and potassium; and (b) a refractory carrier member on which the catalytic trap material is coated, the carrier member being selected from the group consisting of a refractory metal, alumina, titania, zirconia, zirconia-alumina, titania-zirconia, titania-alumina, lanthana-alumina, baria-zirconia-alumina, niobia-alumina, and silica-leached cordierite.
In accordance with yet another aspect of the present invention, the catalytic trap for the above-described purpose comprises the following components: (a) a catalytic trap material comprising (i) a refractory metal oxide support having dispersed thereon a catalytic component effective for promoting the reduction of NOx under stoichiometric or rich conditions of the exhaust stream, and (ii) a NOx sorbent effective for adsorbing NOx under lean conditions of the exhaust gas stream and desorbing NOx under stoichiometric or rich conditions of the exhaust gas stream, the NOx sorbent comprising one or more basic oxygenated compounds of an alkali metal selected from the group consisting of lithium, sodium and potassium present in an amount sufficient to provide, after reaction with the aforesaid basic oxygenated compounds of all silica components present in the catalytic trap, an excess of the basic oxygenated compounds of at least about 0.1 g/in3, e.g., an excess of from about 0.1 to 2.5 g/in3 of the basic oxygenated compounds, calculated as M2O, where M=Li, Na or K; and (b) a refractory carrier member on which the catalytic trap material is coated.
In aspects of the present invention, the alkali metal comprises potassium, so that the basic oxygenated compound is a basic oxygenated compound of potassium.
In accordance with yet another aspect of the present invention, the catalytic trap comprises a refractory carrier member which is inert to the aforesaid basic oxygenated compounds, e.g., a refractory carrier member which is inert to basic oxygenated compounds of potassium, on which is coated a catalytic trap material which is substantially free of silica components. The catalytic trap material comprises a discrete, first layer of catalytic trap material and a discrete, second layer of catalytic trap material overlying the first layer.
Another aspect of the present invention provides that the first layer comprises a first NOx sorbent comprising a first potassium oxygenated compound and a first catalytic component, and the second layer comprises a second NOx sorbent comprising a second potassium oxygenated compound and a second catalytic component.
In aspects of the present invention, the catalytic component is optionally selected from the group consisting of one or more of palladium, platinum and rhodium catalytic components.
For example, one aspect of the layered embodiment of the invention provides for the first catalytic component to comprise a platinum catalytic component and the second catalytic component to comprise a platinum catalytic component, a rhodium catalytic component and a palladium catalytic component.
In another aspect of the layered embodiment of the present invention, (a) the first catalytic component comprises a platinum catalytic component and the first layer further comprises lanthanum, barium and zirconium components, and (b) the second catalytic component comprises a platinum catalytic component, a palladium catalytic component and a rhodium catalytic component, and the second layer further comprises barium and zirconium components.
A broad aspect of the present invention provides for the carrier member to comprise a refractory material which is substantially free of silica components. For example, the carrier member may comprise a refractory metal, e.g., stainless steel, titanium or Fecralloy, or refractory metal oxides, including ceramic-like materials, e.g., alumina, titania, zirconia or silica-leached cordierite. Generally, the refractory metal may be any suitable refractory metal or alloy or refractory oxide or mixed oxide material which is inert to basic oxygenated compounds of one or more of lithium, sodium, and potassium. The carrier member may also comprise a combination of two or more of these materials, e.g., alumina and titania.
The NOx sorbent may further comprise one or more basic oxygenated compounds of one or more metals selected from the group consisting of (a) alkali metals other than lithium, sodium or potassium, (b) alkaline earth metals and (c) rare earth metals, although the latter are not preferred. For example, the NOx sorbent may further comprise one or more basic oxygenated compounds of one or more metals selected from the group consisting of magnesium, calcium, barium, strontium, and cesium.
A particular aspect of the present invention provides for the carrier member of the catalytic trap to have a longitudinal axis, a front face and a rear face, and a plurality of parallel gas-flow passages extending longitudinally therethrough and connecting the front and rear faces of the carrier member. The gas-flow passages are defined by walls on which the catalytic NOx sorbent is coated, and the NOx sorbent comprises one or more basic oxygenated compounds of lithium, sodium or potassium disposed only in a first longitudinal segment of the carrier member, that is, between one of the rear and front faces of the carrier member and an intermediate point along the longitudinal axis thereof (The distance from the front face of the carrier to the intermediate point may comprise from about 20 percent to 80 percent of the length of the carrier along its longitudinal axis.) In this way, the basic oxygenated compound or compounds of lithium, sodium or potassium, e.g., the basic oxygenated compounds of potassium, are excluded from a second longitudinal segment of the carrier member lying between either the front or the rear face of the carrier member and the said intermediate point.
A further aspect of the present invention provides the catalytic trap in combination with a treatment catalyst disposed upstream of the catalytic trap relative to the exhaust gas stream, the treatment catalyst being effective at least to promote under oxidation conditions the oxidation of hydrocarbons to CO2 and H2O.
A method aspect of the present invention provides a method of treating an exhaust gas stream containing NOx to abate the NOx content of the stream. The method comprises maintaining the stream under alternating periods of (1) lean and (2) stoichiometric or rich operation and contacting the stream during the periods of both lean and stoichiometric or rich operation with a catalytic trap material as described above at conditions whereby at least some of the NOx in the exhaust gas stream is adsorbed by the catalytic trap material during the periods of lean operation and is released from the catalytic trap material and reduced to nitrogen during the periods of stoichiometric or rich operation. Contact temperatures may be from about 200xc2x0 C. to 650xc2x0 C., e.g., about 350xc2x0 C. to 650xc2x0 C. (During stoichiometric or rich conditions, however, the catalytic trap may be exposed to higher temperatures, on the order of about 650xc2x0 C. to 850xc2x0 C.)
Another method aspect of the present invention provides that the exhaust gas stream contains hydrocarbons and the method further comprises contacting the exhaust gas stream under oxidizing conditions with a catalyst effective to promote oxidation of hydrocarbons, whereby to oxidize hydrocarbons contained therein, prior to contacting the exhaust gas stream with the catalytic trap.
Other aspects of the present invention are set forth in the appended drawings and in the detailed description set forth below.
Reference herein and in the claims to a xe2x80x9ccarrierxe2x80x9d (sometimes referred to as a xe2x80x9ccarrier memberxe2x80x9d or xe2x80x9ccarrier substratexe2x80x9d) or other material which is xe2x80x9cinertxe2x80x9d to the basic oxygenated compounds of one or more of lithium, sodium and potassium means a carrier or material which is substantially inert to reaction with such basic oxygenated compounds under the conditions encountered by aging or utilization of the catalytic traps of the present invention, including fuel-cut aging cycles at temperatures up to about 900xc2x0 C. (The term xe2x80x9cfuel-cut agingxe2x80x9d is explained below.) Such reaction would render the NOx sorbent (the aforesaid basic oxygenated compound or compounds) significantly less effective in fulfilling its role in NOx reduction. Similarly, reference herein and in the claims to a carrier which is xe2x80x9cpotassium-inertxe2x80x9d means a carrier which is substantially inert to reaction under the described conditions with basic oxygenated compounds of potassium.
As used herein and in the claims, an xe2x80x9coxygenated metal compoundxe2x80x9d means a compound including one or more metals and oxygen. For example, the aforesaid basic oxygenated metal compounds may comprise one or more of a metal oxide, a metal carbonate and a metal hydroxide.
Reference herein and in the claims to xe2x80x9ccomponentxe2x80x9d or xe2x80x9ccomponentsxe2x80x9d with reference to catalytic components such as palladium, platinum or rhodium catalytic components means the metal or metals, as the element, alloy or compound, in catalytically effective form, e.g., usually as the element or an alloy. Similarly, reference herein and in the claims to metal xe2x80x9ccomponentsxe2x80x9d comprising NOx sorbents means any effective NOx-trapping forms of the metals, e.g., oxygenated metal compounds such as metal hydroxides, mixed metal oxides, metal oxides or metal carbonates.
The quantities of components of the catalytic material are expressed herein in units of weight per unit volume, specifically, grams per cubic inch (xe2x80x9cg/in3xe2x80x9d) and grams per cubic foot (xe2x80x9cg/ft3xe2x80x9d). This system of nomenclature accommodates voids in a carrier member such as the carrier member having a plurality of parallel, fine gas-flow passages extending therethrough, on the walls of which the catalytic NOx sorbent is coated. The nomenclature would similarly accommodate the voids contained in an embodiment wherein the catalytic NOx sorbent is coated onto beads of a catalytically inert material, the inert beads and the interstices between them providing voids in the catalytic trap. Concentrations (xe2x80x9cloadingsxe2x80x9d) in the trap member of catalytic metals such as Pd, Rh and Pt are given on the basis of the elemental metal and are expressed as, e.g., 200 g/ft3 Pd, 90 g/in3 Pt, etc. Loadings of NOx sorbents are similarly given on a weight per volume basis, but as grams per cubic inch (xe2x80x9cg/in3xe2x80x9d), and calculated on the basis of the following oxides: Li2O, Na2O, K2O, Cs2O, MgO, CaO, SrO and BaO. The coating of the catalytic NOx sorbent on the carrier member is sometimes referred to as a xe2x80x9cwashcoatxe2x80x9d because the carrier member is typically coated with an aqueous slurry of particles of the solids, e.g., the refractory metal oxide support, and the slurry coating is then dried and heated (calcined) to provide the washcoat.
Reference herein and in the claims to the use of xe2x80x9cdispersionsxe2x80x9d or the like of precursor compounds in a liquid includes the use of solutions or other dispersions in a liquid vehicle of precursor compounds and/or complexes.
Reference herein to a mixed metal oxide means an oxide which contains two or more metals, such as barium zirconate, barium titanate, barium aluminate, etc.
As used herein and in the claims, reference to xe2x80x9csilica componentsxe2x80x9d means and includes silica and silicates; reference to a material being xe2x80x9csubstantially freexe2x80x9d of silica components means that the quantity of silica components present, if any, is too little to noticeably adversely affect the efficacy of the catalytic trap material.